This invention relates in general to hydraulic steering systems for straddle-type cranes and, in particular, to a synchronization system for automatically synchronizing the steering system utilized on such cranes. More specifically, but without restriction to the particular use which is shown and described, this invention relates to a hydraulic steering synchronization system for use on straddle-type cranes wherein synchronization between the steering wheel on either side of the crane may be resynchronized automatically or upon operator actuation of the system and steering the wheels through a normal steering limit cycle.
For convenience of illustration, the hydraulic steering synchronization system will be described with reference to its use on a sling-rigged straddle crane which is especially suitable for use in handling watercraft. However, it is to be understood that the steering synchronization system is equally applicable to other straddle-type cranes used for industrial or commercial purposes, such as freight loading, containerized shipping for materials handling. In all of such straddle-type cranes, a hydraulic steering system is used that includes a hydraulic cylinder and mechanical linkage on each side of the crane to control the steering angle of steerable tires and to provide accurate tracking of the non-steering or travel tires. Such a steering system is designed so that the non-steering or travel wheels of the straddle crane, track the travel of the steering wheels when traveling in a straight-line motion, or through a steering/cornering motion. This type of steering system is referred to by those skilled in the art as the "Ackerman" steering system.
Since the steering wheels are not physically connected or mechanically linked one to the other, when a differential exists in the hydraulic system of the left and right steering cylinders, actuation of these cylinders and the resultant movement of the steering wheels, is no longer synchronized. As a result, tire scuffing and scrubbing occurs which not only causes unnecessary and excessive tire wear, but can create a lateral force in the frame structure which could create a safety hazard.
Heretofore, when the steering wheels were caused to go out of synchronized alignment, or found to be so, the machine operator or a mechanic was required to manually bleed the hydraulic cylinders in order to resynchronize the steering system and again place the steering wheels in proper alignment. The operator or mechanic was required to manually operate needle-type cross-circuit valves to bleed hydraulic fluid from one hydraulic cylinder to the other. Not only is this manual operation cumbersome and time consuming, but frequently machine operators did not resynchronize the steering system until the misalignment had become quite excessive. Since the crane had to be taken out of operation to manually resynchronize the steering system, in busy seasonal types of operations such misalignment frequently became quite excessive.